Process and apparatus for applying silver films



July 23, 1963 E. F. HARACZ 3,098,756

PROCESS AND APPARATUS FOR APPLYING SILVER FILMS Filed Feb. 4, 1959 2 Sheets-Sheet 1 INVENTOR. Edward jf'ffara c m wgm,

July 23, 1963 E- F. HARACZ PROCESS AND APPARATUS FOR APPLYING SILVER FILMS Filed Feb. 4, 1959 2 Sheets-Sheet 2 INVENTOR. Z Hn4z2'alEHdzac7,

PW @Zy/Q JiiZryqya-h 3,098,756 PROCES AND APPARATUS FOR APPLYING SILVER FILMS Edward F. Haracz, 161 Pershing Road, Clifton, N .J Filed Feb. 4, 1959, Ser. No. 791,184 21 Claims. (Cl. 11735) This invention relates to the application of a film to a surface. It was occasioned by the need for improvement in the application of a film of silver to the surface of glass whereby to form a mirror, and it will be described with reference to this application. The invention can, however, be employed for other purposes such as for example the application of silver to a surface to form a printed circuit.

Mirrors are commonly manufactured by passing glass plates by means of a conveyor through a silver film applying zone disposed in the path of the conveyor. In the film applying zone, a liquid conduit means, which includes a tube for conveying silver salt-containing solution and a tube for conveying a reducing solution, is moved traversely back and forth across the path of the plates, and silver salt solution and reducing solution are directed onto the upper surface of the plates, so that they combine on the plates. In the resulting admixture, a reaction occurs whereby silver is precipitated and this deposits on the plates and provides the desired mirroring effect.

Due to the relative movement of the glass plates and the liquid conduit means, the spent liquid medium resulting from the precipitation of silver and which contains a sludge formed as an incident of the reduction reaction, flows onto portions of the plates which are subsequently to receive the solutions and have applied thereto silver film. It has been recognized heretofore that the effectiveness of the above-described manufacturing method depends on the effectiveness with which spent solution can be removed from areas of the plates prior to application thereto of the silver salt solution and reducing solution. Various means have been proposed for the removal of spent solution. So far as is known to me, however, none of the proposed means are suitably satisfactory. It is, therefore, an object of the invention to provide an improved manner for the removal of spent liquid medium.

A further object of the invention is to provide for the removal of spent liquid medium which is complementary with other operations involved in the mirroring process in the manner that removal of spent liquid medium is effected without reducing the effectiveness of other operations. Thus, the spent medium is to be removed in a manner such that recovery of unused silver therefrom can be high, the contamination of the area of the mirroring operation can be low, and further so that the equipment employed in aid of spent medium removal will not interfere with the mirroring as for example by contaminating either the silver salt or reducing solution with oil, dirt, grease, etc.

The invention otherwise provides improvement in the manufacture of mirrors as described above. Heretofore, the practice in the art has been to make up batches of the silver salt containing solution and the reducing solutions by suitably diluting concentrates with water. The batches are stored in tanks and the solutions are withdrawn from tankage as required. I have discovered that when operating in this manner, the film of silver deposited varies in character during the operation and that this variation can be significantly reduced by continuously forming the silver salt containing solution and the reducing solution from concentrates and water rather than making these solutions up batchwise. Accordingly, it is an object of the invention to provide for operation of the method de- Patented July 23, 1963 scribed hereinbefore in a manner such that silver films of more uniform character are obtained.

The invention will be further described by reference to the accompanying drawing wherein an embodiment of the invention is depicted. The embodiment set forth in the drawing is merely representative and various alternative embodiments are within the scope of the invention.

In the drawings:

FIG. 1 is a schematic representation and elevation view of a mirroring device provided with the improvements of the invention;

FIG. 2 and FIG. 3 are cross-sectional views taken respectively, on line Z2 and 33 in FIG. 1;

FIG. 4 is an enlarged view of a portion of the device shown in FIG. 2 and FIG. 3 and further indicating fiow of fluid streams when the device is in operation; and

FIG. 5 is view taken along line 55 in FIG. 4.

According to the invention improvement in the removalof spent liquid medium from the surface to be mirrored is realized by providing for directing a fluid stream, preferably a gas stream, onto the surface adjacent and in advance of liquid medium flowed onto the surface. Thereby the course of the liquid medium, as is determined by movement of a liquid conduit means over the surface, is cleared of spent liquid medium and thus is prepared for receiving liquid medium in condition to deposit silver onto the surface.

When the liquid conduit means is moved over a course on the surface extending back and forth on the surface first toward one side thereof and then toward an opposite side thereof, there is preferably provided a first and second gas conduit means, each mounted for travel with the liquid conduit means and disposed, respectively, one on one side of the liquid conduit means in the direction of the course on the surface and the other on the other side of the liquid conduit means in the direction of the course. Advantageously, there is provided a valve means for alternately communicating each of the gas conduit means with a supply of compressed gas and means for controlling the valve means whereby the gas conduit leading the liquid conduit means is maintained in communication with the compressed gas supply by alternately communicating each gas conduit means with the compressed gas supply in synchronism with the movement of the liquid conduit means back and forth over said surface. Thus a first gas stream can be directed onto the course adjacent and in advance of liquid medium from the liquid conduit means during travel of the liquid conduit means toward one side of the surface and, during travel of the liquid conduit means toward the other side of the surface, the flow of said first stream of gas can be maintained shut off and a second stream of gas can be directed onto the course in similar manner during travel of the liquid conduit means toward the other side of the surface.

The liquid means can comprise two tubes disposed in side by side relation, each of which is provided with nozzles positioned at spaced intervals therealong and arranged so that fluid discharge from the nozzles of one of the tubes intercepts fluid discharged from the nozzles of the other of the tubes. The gas conduit means, one positioned on each side of the liquid tubes, can each have outlets for gas disposed so that gas discharged therefrom will flow into the path of the nozzles of the liquid discharge tubes. Preferably the gas outlets are such that the gas stream flows from the gas conduit means outwardly of the liquid tubes of the liquid conduit means. Advantageously, the outlets in each of the gas conduit means is in the form of a continuous longitudinally extending slit of length such that the spent liquid medium in advance of the liquid conduit means is cleared from the surface over the entire length of the liquid conduit means.

With respect to forming a film of more uniform character, according to the invention, continuous formation of solution used in the mirroring operation is employed rather than batching. Heretofore, the practice has been to form a batch, e.g. 100 to 200 gallons of alkaline ammoniated silver salt solution by diluting with water a concen trated ammoniated silver nitrate solution and a concentrated alkaline solution, and to form a batch, e.g. 100 to 200 gallons, of a reducing solution by diluting with water a concentrated aqueous reducing solution such as a dextrine solution. According to the invention, a silver saltcontaining liquid is formed continuously during the flowing of the silver salt-containing solution and the reducing solution onto the surface to be coated with a silver film by admixing a concentrated aqueous silver salt solution, e.g. a concentrated ammoniated aqueous silver nitrate solution, and an aqueous medium, e.g. an alkaline aqueous medium. Desirably, the reducing solution is also formed continuously and this can be done by admixing a concentrate of the reducing agent and water continuously during the flowing of the solutions onto the surface to be silvered.

The improvement which can be realized by the continuous operation of the invention rather than the batch operation of the prior art can be demonstrated by comparing light transmission of mirrored glass samples taken at the beginning and the end of runs. Thus, when continuous operation is employed, transmission is substantially the same at the beginning of a run as at the end whereas where batchwise operation is employed light transmission will commonly vary by 33 as between window glass or stock mirror samples taken at the beginning and end of a run and tested by holding a 75 watt bulb about one foot away from the sample and measuring transmission in candle power with a photo electric meter held opposite the meter from the bulb. Also, the improvement can be demon strated by comparing the weight per unit area of film deposited per unit area. Thus, by the prior art method the deposit at the beginning of a run will commonly be about 18% greater than at the end of a run, Whereas variation employing the method of the invention is plus or minus about 2 /2 Heretofore, the liquid streams have commonly been directed onto the surface to be mirrored as aerated sprays. According to the preferred embodiment of the invention, each of the liquid streams is a continuous stream. The use of gas streams independent of the liquid streams for clearing the surface of spent liquid medium permits operation in this manner, and such operation has the advantage of reducing contamination of the surrounding area, permitting recovery of increased amounts of silver from spent solution and further permits the use of a relatively inexpensive high pressure, non-lubricated, centrifugal blower rather than a carbon ring, carbon piston, oil-less type reciprocating compressor as is commonly used when the liquid streams are aerated.

Further, the employment of continuous liquid streams is compatible with the continuous formation of solution according to the invention. Since air or compressed gas is not mixed with solution to form an aerated spray, the likelihood of the air or gas lessening the effectiveness of solution is greatly minified.

Considering now the particulars of the embodiment shown in the drawing, the apparatus there shown comprises a conveyor made up of endless belts 11 mounted on rollers 13 which in turn are mounted on supports 14. Glass plates 15 are positioned on the conveyor and are moved thereby, in the direction indicated in FlG. l by the arrow 16, through a silver film applying zone 17 contained Within the enclosure 18. The conveyor advances the plates through the film applying zone 17 at, for example, about six feet per minute, and as the plates move through the zone, the liquid conduit means 20 moves back and forth transversely of the path of the conveyor and directs liquid medium onto the upper surface of the plates. Silver is precipitated from the liquid medium, deposits on the plates and thus forms the mirrors.

The liquid conduit means 20, as can be best seen in The car is moved over the track by endless belt 37 (see FIG. 2) which is driven through belt drive 36 by reversing motor 38. Movement of the car is controlled by switches 38 and 39 which are arranged to be actuated by contact of the car with switch buttons 40 and 41 in the manner that as the car contacts either of the bottoms 40 or 41 a signal is sent through power lines 42 or 43, respectively, to switch 45 which in turn sends a signal through power lines 46 to the motor 38 and reverses the rotation of the motor, which in turn reverses the direction of travel of the car on the track 33. The spaced of the car can be, for example, about 300 feet per minute. The speed of the car and the conveyor are not critical and those employed in the art are suitable.

For clearing the glass plates 15 of spent solution, a gas conduit 47 is mounted on one side of the liquid conduit means 20 and in the direction of the path followed by the liquid conduit means, and a gas conduit 48 is mounted on the other side of the liquid conduit means and in the direction of the same path. The gas conduits 47 and 48 are connected, respectively, by gas connecting pipes 49 and 50 to the flexible hose 51, which in turn is connected by gas supply pipe 52 to centrifugal air blower 53. At the junction of the gas connecting pipes 49 and 5t} and the flexible hose 51, there is installed a valve 55 which is secured to the car 32 and thus provides -a mounting for the gas conduits 47 and 48 so that they travel with the liquid conduit means 20'. The valve 55 is a two position valve adapted to direct compressed air from the blower 53 to one of the gas conduits 47 or 43 and to simultaneously maintain the supply of compressed air to the other of the gas conduits shut olf. The valve is provided with a solenoid 56 which is connected by lines 57 to switch 45, and the hook-up is such that the gas conduit leading the liquid conduit means is supplied wtih compressed air while the supply to the gas conduit trailing the liquid conduit means is shut off. By operation of the switch 45, which is controlled by switches 38 and 39, when the direction of travel of the car 32 changes, the valve position changes so that the described air flow in relation to the movement of the liquid conduit means is maintained.

The operation of the liquid conduit means and the gas conduit means is indicated in FIG. 4. In this view, the liquid conduit means 20 is moving in the direction indicated by the arrow 60. A continuous stream 61 of the silver salt-containing solution issues from silver salt-containing solution tube 21 and a continuous stream 62 of reducing solution issues from reducing solution tube 22. These streams intercept each other at the surface of the glass plate 15, admix and form a liquid medium 63. Due to chemical reaction, silver precipitates from the liquid medium and deposits on the glass plate as a silver film 64. During previous travel of the liquid conduit means 28 over the glass plate 15 spent liquid medium 66 which contains a sludge, has accumulated along the course on which the liquid medium is being deposited from the liquid conduit means. This spent solution is removed from the course by air stream 67 which issues from the gas conduit 48 disposed in advance of the liquid conduit means 20. The supply of air to the gas conduit 47, which trails the liquid conduit means is shut :oif and hence spent or partially spent liquid medium is not moved along the course over the surface of the plate followed by the liquid conduit means. Maintaining air flow to the leading gas conduit and maintaining air flow to the trailing gas conduit shut off is the preferred manner of operation.

The construction of the liquid tubes 21 and 22 and the gas conduits 47 and 48 is indicated in FIG. 5. The liquid tubes are provided with nozzles 70 positioned at spaced intervals therealong. The nozzles can be Spraying Systems, Inc. #8002. The nozzles can be positioned to provide spaced continuous sprays of about 71() inches length in the direction of the tubes. The air outlets in the gas conduits 47 and 48 can advantageously be in the form of accurately machined longitudinally extended slits 63 which are of length suitable to clear spent solution from the path traversed by the nozzles of the liquid tubes. The slits can be about in width. Desirably the outlets in the gas conduit tubes are arranged to discharge the air outwardly of the liquid conduit means 20, as is indicated in FIG. 4 and a suitable angle for the gas stream is about 5 degrees from the verticle. Advantageously, the gas conduits can be constructed so that this angle is adjustable.

The enclosure 18 for the film forming zone 17 is provided with an outlet 7 0 through which spent solution collected in the bottom of the enclosure can be withdrawn. Advantageously, connection is made to the outlet 7 t} to provide a downdraft in the enclosure. This arrangement reduces contamination of the surrounding area and permits increased recovery of silver from spent solution.

The formation of silver salt-containing solution and reducing solution according to the invention is indicated in FIG. 1. A concentrated aqueous ammonia silver solution is contained in a carboy 7 1, a concentrated alkaline solution is contained in a carboy 72 and a concentrated aqueous dextrine solution is contained in carboy '73. Deionized Water, aqueous ammonia silver concentrate and alkaline solution are each pumped by variable displacement pumps 74 into silver salt-containing solution supply line 23, and deionized water and concentrated aqueous dextrine solution are each pumped by variable displacement pumps 74 into reducing solution supply line 27. The pumping operations are maintained continuously during the silvering operation so that the solutions employed are formed continuously. The variable displacement pumps permit suitable proportioning of the various streams, and, to further permit accurate control of the flow of the solutions, reach of the solution supply lines is provided with a constant pressure regulator valve 75. Preferably, a flow meter 74' is provided for each pump. Ifdesired venturi-type iejectors, employing water as the motive stream to syphon the concentrates, can be employed in place of pumps 74.

The continuous operation offers the advantages over batch operation that, when operating in the former manner, tanks are not required, and less labor is required for operation.

The ammoniated silver nitrate concentrate can be a concentrate such as is commonly used in the art. For example, it can be an aqueous solution of ammonia and silver nitrate in the propontions of silver salt to ammonia commonly employed in the art and near saturated with respect to silver nitrate. It can also be an anhydrous solution. Suitable compositions for the concentrate are, on a weight basis, about 25-32 parts of silver nitrate, about 50-60 parts ammonia and 2530 parts water. Normally, the amount of water will not exceed the amount of ammonia.

The alkaline solution can be a concentrate such as is commonly used in the art. For example, it can be an aqueous sodium hydroxide solution of composition about 1.75 to 2.75 pounds of sodium hydroxide per gallon of water. A typical solution contains about 2.2 pounds of sodium hydroxide per gallon of water.

The reducing solution can be any reducing concentrate such as is commonly used in the art. Advantageously, it can be an acidified dentrine solution of composition containing- 7-10 pounds, for example 8, pounds of dextrine 6 per gallon of water, a small amount of formaldehyde, for example about 2.7-3.5 ounces per gallon, and a small amount of acid, for example about one-fourth ounce per gallon of 98% sulfuric acid.

The concentrate can be dispensed from carboys in which they are transported, the carboys being vented to air for dispensing in ways well known in the art.

Having now described my invention, what it is desired to claim is:

1. In combination with a device for applying a film to a surface by directing a liquid medium containing film forming material onto the surface and depositing the film forming material from the medium on the surf-ace and leaving spent liquid medium on the surface, and comprising liquid conduit means for directing the liquid medium onto the surface, guide means for moving the liquid conduit means relative to the surface through a course over the surface whereby spent liquid medium flows onto a subsequently to be traversed portion of the course, fluid conduit means mounted adjacent to said liquid conduit means for travel therewith for directing a stream of fluid onto at least part of said course adjacent and in advance of liquid medium directed onto the course by the liquid conduit means and thereby clean said part of the course in preparation thereof for receiving liquid medium from the liquid conduit means.

2. In combination with a device for applying a film to a surface by directing a liquid medium containing film forming material onto the surface and depositing the film forming material from the medium on the surface and leaving spent liquid medium on the surface, and comprising liquid conduit means for directing the liquid medium onto the surface, guide means for moving the liquid conduit means relative to the surface through a course extending back and fonth over the surface whereby spent liquid medium flows onto a portion of the course subsequently to be traversed by the liquid conduit means, a first gas conduit means mounted adjacent the liquid conduit means for travel therewith and on one side of the liquid conduit means along said course for directing a stream of gas onto said course as the liquid conduit means moves along said course toward the first gas conduit means, and a second gas conduit means mounted adjacent the liquid conduit means for travel therewith and on the other side of the liquid conduit means along said course for directing a stream of gas onto said course as the liquid conduit means moves along said course toward the second gas conduit means, whereby said course can be cleared of said medium in advance of the liquid conduit means.

3. The combination according to claim 2 and including valve means for alternately communicating each of the gas conduit means with a supply of compressed gas, and control means for the valve means, said control means being effective to maintain the gas conduit leading the liquid conduit means in communication with the compressed gas supply by alternately communicating each gas conduit means with the compressed gas supply in synchronism with the movement of the liquid conduit means back and fonth over said surface.

4. The combination according to claim 2, said liquid conduit means comprising two tube-s disposed in side by side relation and each having nozzles for directing a stream of liquid onto the surface positioned at spaced intervals therealong, and each gas conduit means comprising a tube having means defining an outlet for directing compressed gas onto said surfaces in the path of each nozzle over said course.

5. The combination according to claim 4, the outlets in the gas conduit means being arranged to discharge the compressed gas outwardly of the liquid conduit means.

6. The combination according to claim 4, and including valve means for alternatively communicating each of the gas conduit means with a supply of compressed gas, and control means for the valve means, said control means being efiective to maintain the gas conduit leading the liq- 7 uid conduit means in communication with the compressed gas supply by alternatively communicating each gas conduit means with the compressed gas supply in synchronism with the movement of the liquid conduit means back and forth over the surface.

7. The combination according to claim 2, said liquid conduit means comprising two tubes disposed in side by side relation and each having nozzles for directing a stream of liquid onto the surface positioned at spaced intervals therealong, and each gas conduit means comprising a tube having means defining a continuous longitudinally extending slit for directing compressed gas onto said surface in the path of the nozzles over said course.

8. The combination according to claim 7, the slit in the tube of each gas conduit means being arranged to discharge the compressed gas outwardly of the liquid conduit means.

9. The combination according to claim 7, and including valve means for alternately communicating each of the gas conduit means with a supply of compressed gas, and control means for the valve means, said control means being effective to maintain the gas conduit leading the liquid conduit means in communication with the compressed gas supply by alternately communicating each gas conduit means with the compressed gas supply in synchronism with the movement of the liquid conduit means back and forth over the surface.

10. In the method of making mirrors by depositing a silver film onto a surface which comprises moving a liquid conduit means relative to the surface over a course on the surface, flowing liquid medium containing film forming material through the liquid conduit means and onto said course, allowing the film forming material to deposit onto the surface from the liquid medium forming spent liquid medium, whereby spent liquid medium flows onto a portion of said course subsequently to be traversed by the liquid conduit means, the improvement which comprises directing a stream of gas onto at least part of said course adjacent and in advance of liquid medium directed onto the course by the liquid conduit means thereby clearing said part of the course of spent liquid medium in preparation thereof for receiving liquid medium from the liquid conduit means.

11. The method according to tclaim 10 wherein said liquid medium comprises a first stream of liquid which contains a reducing agent, said first and second streams of liquid each being a continuous liquid stream and being directed onto the surfacee from the liquid conduit means so. that they are admixed on the surface.

12. In the method of making mirrors by depositing a silver film onto a surface which comprises moving a liquid conduit means relative to the surface over a course extending back and forth on the surface first toward one side thereof and then toward an opposite side thereof, flowing liquid medium containing film forming material through the liquid conduit means and onto said course, allowing the film forming material to deposit onto the surface from the liquid medium forming spent liquid medium whereby spent liquid medium flows onto a portion of said course subsequently to be transversed by the liquid conduits means, the improvement which comprises directing a first stream of gas into said course adjacent and in advance of liquid medium directed onto the course by the liquid conduit means during travel of the liquid conduit means toward one side of the surface and, during travel of the liquid conduit means toward the other sides of the surface, maintaining the flow of said first stream of gas shut off and directing a second stream of gas onto said course adjacent and in advance of liquid medium directed onto the course, whereby said course is placed of spent liquid medium is preparation thereof for receiving liquid medium from the liquid conduit means.

13. The method of claim 12 wherein said liquid me- 8 dium comprises a first stream of liquid which contains a silver salt and a second stream of liquid which contains a reducing agent, said first and second streams of liquid each being a continuous liquid stream and being directed onto the surface from the liquid conduit means so that they are admixed on the surface.

14. In the method of making mirrors by depositing a silver film onto a surface which comprises moving a liquid conduit means relative to the surface over a course on the surface, flowing onto said course from said conduit means a first stream of liquid which contains a silver salt and a second stream of liquid which contains a reducing agent onto said course so that said streams are admixed on the surface and form a liquid medium, and allowing reducing agent to reduce silver salt present in the liquid medium and precipitate silver on the surface, and form a spent liquid medium, whereby spent liquid medium flows onto a portion of said course subsequently to be traversed by the liquid conduit means, the improvement which comprises forming said first stream continuously during the flowing of said first and second streams of liquid onto the surface by admixing a concentrated aqueous silver salt solution and an aqueous medium, flowing each the first and second stream onto the surface as a continuous liquid stream, and directing a stream of gas onto at least part of said course adjacent and in advance of liquid medium directed onto the course by the liquid conduit means thereby clearing said part of the course of spent liquid medium in preparation thereof for receiving liquid medium from the liquid conduit means.

15. The method of claim 14 wherein the concentrated silver salt solution is ammoniated and the aqueous medium with which it is admixed is aln alkaline aqueous medium.

16. The method of claim 14 wherein said second stream of liquid is formed continuously during the flowing of said first and second streams of liquid onto the surface by admixing a concentrated solution of the reducing agent and water.

17. In the method of making mirrors by depositing a silver film onto a surface comprising flowing a first stream of liquid which contains a silver salt and second stream of liquid which contains a reducing agent onto the surface so that said stream-s are admixed on the surface, and allowing the reducing agent to reduce silver salt present in the admixture and precipitate silver onto the surface, the improvement which comprises forming said first stream continuously during the flowing of said first and second streams of liquid onto the surface by admixing in predetermined proportion a concentrated silver salt solution and an alkaline aqueous medium in a flowing stream of water.

18. The method of claim 17 wherein said second stream of liquid is formed continuously during the flowing of said first and second streams of liquid onto the surface by admixing a concentrated solution of the reducing agent in a flowing stream of water.

19. The method of claim 18, wherein the concentrated silver salt solution is ammoniated.

20. The process of producing a metallic layer on a support which comprises moving said support while maintained in a horizontal position, moving transverse to the direction of movement of said support in one direction across said support a spray of a flushing fluid to project said flushing fluid onto said support and a spray of metal depositing solution which is' projected onto the area contacted by said fluid immediately following the contact of said area by said fluid, thus forming a thin metal layer on said support covered by spent solution, repeating the last mentioned step at least once for said metal depositing solution to form a plurality of layers of metal on each unit area of said support, and eflecting the removal of said spent solution with said flushing fluid 9 before the said repeating of the spraying of said metal depositing solution.

21. The process of producing a metallic layer on a support which comprises moving said support while main tained in a horizontal position, moving transverse to the direction of movement of said support across said support a spray of metal depositing solution, thus forming a thin metal layer on said support covered by spent solution, repeating the last mentioned step at least once to form a plurality of layers of metal on each unit area of said support, and effecting removal of said spent solution with a spray of flushing fluid before each repetition of the spraying of said metal depositing solution.

References Cited in the file of this patent UNITED STATES PATENTS 

14. IN THE METHOD OF MAKING MIRRORS BY DEPOSITING A SILVER FILM ONTO A SURFACE WHICH COMPRISES MOVING A LIQUID CONDUIT MEANS RELATIVE TO THE SURFACE OVER A COURSE ON THE SURFACE, FLOWING ONTO SAID COURSE FROM SAID CONDIT MEANS A FIRST STREAM OF LIQUID WHICH CONTAINS A REDUCING AGENT SECOND STERAM OF LIQUID WHICH CONTAINS A REDUCING AGENT ONTO SAID COURSE SO THAT SAID STREAMS ARE ADMIXED ON THE SURFACE AND FORM A LIQUID MEDIUM, AND ALLOWING REDUCING AGENT TO REDUCE SILVER SALT PRESENT IN THE LIQUID MEDIUM AND PRECIPITATE SILVER ON THE SURFACE, AND FORM A SPENT LIQUID MEDIUM, WHEREBY SPENT LIQUID MEDIUM FLOWS ONTO A PORTION OF SAID COURSE SUBSEQUENTLY TO BE TRAVERSED BY RHE LIQUID CONDUIT MEANS, THE IMPROVEMENT WHICH COMPRISES FORMING SAID FIRST STREAM CONTINUOSULY DURING THE FLOWING OF SAID FIRST AND SECOND STREAMS OF LIQUID ONTO THE SURFACE BY ADMIXING A CONCENTRATED AQUEOUS SILVER SALT SOLUTION AND AN AQUEOUS MEDIUM, FLWOING EACH THE FIRST AND SECOND STREAM ONTO THE SURFACE AS A CONTINUOUS LIQUID STREAM AND DIRECTING A STREAM OF GAS ONTO AT LEAST PART OF SAID COURSE ADJACENT AND IN ADVANCE OF LIQUID MEDIUM DIRECTED ONTO THE COURSE BY THE LIQUID CONDUIT MEANS THEREBY CLEARING SAID PART OF THE COURSE OF SPENT LIQUID MEDIUM IN PREPARATION THEREOF FOR RECEIVING LIQUID MEDIUM FROM THE LIQUID CONDUIT MEANS. 